This fall, a dozen colleges across the country have introduced a controversial new teaching tool called course casting, aimed at supplementing—and in some cases replacing—large, impersonal lectures. Although it has been around for less than a year, course casting has become as popular as a keg party on homecoming weekend. Students at Purdue University have downloaded 40,000 lectures since the start of the semester—not bad for a school with an enrollment of 38,000.